Scientists from Lehigh University, with help from the U.S. National Science Foundation, used bone imaging to study how fractures rebuild and model the mechanical properties of healing bone. This helped them make progress on making a virtual biomechanical test that measures how well fractures are healing.
You need to come up with some way to treat callus differently when modelling healing bone. But the mechanical properties of callus still aren’t well understood.
– Brendan Inglis, lead author.
A callus is a heterogeneous tissue, containing multiple values for density and stiffness. Inglis added that the goal is to save patients time, money, and irritation and the model could help doctors better treat bone fractures and diagnose non-union, which occurs during failed mending fractures.
The study describes how the healing zone, which consists of a mixture of soft and hard tissue, determines the mechanical rigidity of the entire bone. Previous models failed to effectively differentiate between bone and callus, the semi-soft material that forms a temporary bridge between fragmented bone fragments, resulting in skewed evaluations of the healing process.
According to the researchers, this approach will account for the variable densities and stiffness of mending bone, resulting in more realistic models. The group employed a model that differentiates between bone and soft tissue.
Once the model has been validated against what was done on a bench test, it can begin to predict various aspects of bone healing behaviour. The more people understand why the healing process fails, the better their chances of developing a tool that will one day help surgeons. As a result, this model provides them with a foothold in translating the work into the clinic.
The development of a clinically relevant, reliable, scalable, non-invasive bone healing test could have a significant impact on the treatment of non-union fractures. Detecting problems with bone healing sooner would allow for earlier intervention when necessary, and potentially better outcomes for the most vulnerable patients.
The problem of material modelling in fracture callus is a major barrier to clinical translation of image-based virtual mechanical testing for addressing this need. Callus is an inherently transient material that lasts only a few months in a living organism before being remodelled. Because the mechanical properties of callus cannot be studied in human cadavers, the understanding of its material mechanics and structural organisation must rely heavily on observations from large animals.
Furthermore, the study’s findings show that the material properties of the tissues in the callus region of ovine long bones are not monolithic and should not be treated as such. This idea resonates strongly with histomorphometric data, which show subzones of different compositions within fracture callus and mineralisation gradients that develop in both space and time.
The dual-zone or piecewise material model used in this study aids in two ways in capturing callus heterogeneity. For starters, it recognises that where the callus contains bone, this tissue should be modelled locally as bone. Second, the dual-zone material model recognises that in early or slow healing, fibrous and cartilaginous soft tissues may constitute a significant portion of the healing zone.
The findings of this study confirm that soft tissues within the callus are not simply low-density bone in terms of structural mechanics. This can be deduced from the fact that assigning their mechanical properties using a bone-derived radiodensity scaling law results in a systematic over-prediction of limb rigidity.
